The present invention relates to a method for calibrating n-port receivers as well as to a receiving apparatus provided with means for supplying a signal to calibrate a n-port receivers comprised in the receiving apparatus. The present application is furthermore directed on a mobile communication device provided with such a receiving apparatus.
When referencing to n-port receivers in the following description, n is an integer which can assume the value of four, five and six. As the case of n equal to six is known in the prior art, the following description is made with reference to a six-port receiver. However, the present invention is by no means limited on the case of n equal to six. The calibration of a n-port receiver is essentially independent of the fact whether n is four, five or six.
Recently it has been shown that a so-called six-port receiving circuit in conjunction with a digital signal processor is capable of performing digital demodulation directly at frequencies ranging from microwave to mm-wave bands. This new direct digital receiver promises reduced receiver complexity, low fabrication requirements and fair performance in providing a cost-effective alternative to the conventional heterodyne structure used in various digital terminals.
FIG. 3b shows schematically the application area of a direct six-port receiver as a partial or complete replacement of a conventional heterodyne receiver structure (FIG. 3a).
FIG. 4 shows the structure of a six-port receiver known from Bossisio, Wu xe2x80x9cA six-port direct digital millimeter wave receiverxe2x80x9d, Digest of 1994 IEEE MTT Symposium, vol. 3, page 1659-1662, San Diego, May 1994.
The six-port technique has been known for its ability to accurately measure the scattering parameters, both amplitude and phase, of microwave networks. Instead of using heterodyne receivers a six-port receiver accomplishes direct measurements at microwave and mm-wave frequencies by extracting power levels at at least three and particularly four of the six ports. The imperfections of the hardware can be readily eliminated by an appropriate calibration procedure. Very accurate measurements can be made of a large dynamic range and wide frequency range. six-port junction receivers consist of passive microwave components such as directional couplers and power dividers as well as diode detectors. The circuit can be easily integrated as MHMIC or MMIC. The known receiver performs direct phase/amplitude demodulation at microwave and mm-wave frequencies. The traditional I-Q block in a receiver is replaced by a six-port phase/frequency discriminator which contains a six-port receiver and a digital signal processing (DSP) unit. The incoming digitally modulated RF-signal is compared with the output of a digital controlled local oscillator 18. Carrier recovery is first performed. The DSP-unit 17 detects the frequency difference of the signals and then controls the local oscillator 18 to track the incoming signal. Once the carrier is recovered the instantaneous phase of the received signal is detected and decoded so as to recover the original modulated data. The maximum data transmission rate is determined mainly by the sampling rate of the A/D-converters 16 and the processing speed of the DSP-unit 17.
Six-port receivers generally require a calibration. One major advantage of the six-port receiver is the ability to cope with non-perfect (non-ideal) RF sub-systems. Calibration procedures extract the imperfections of the six-port-hardware. The results of the calibration are in general complex coefficients. Those complex coefficients multiplied with the measured power levels at different ports are required to calculate the (relative) amplitude and (relative) phase of the incoming signal of the receiver. The relative magnitude and the relative phase are related to the coherent or non-coherent detection of the signal.
The calibration parameters are in general non-time dependent or very slowly changeable with time. Theoretically, the calculation of the correction calibration parameters is required to be performed only once. However, in practice they should be performed every time a very large time period has elapsed, which very large time period should be evaluated on case by case basis. The changing of the RF parameters depends on the environmental conditions as well as the manufacturing imperfections.
From the state of the art different calibration techniques for a six-port receiver structure have been proposed. They are generally applied for six-port structures for net work measurement issues and such as usage of hardware termination for calibration like known loads, shorts and sliding shorts.
Such calibration techniques are known from G F Engen, xe2x80x9cCalibrating the six-port reflectometer by means of sliding terminationxe2x80x9d, IEEE Trans. Microwave Theory Technique, vol. 26., pages 987-993, December 1978 and U. Stumper, xe2x80x9cFinding initial estimates needed for the Engen method of calibrating single six-port reflectometersxe2x80x9d, IEEE Trans. Microwave Theory Technique, vol. 38, pages 951-957, July 1990 as well as F. Wiedmann, B. Huyert, E. Bergeault and L. J. Allet xe2x80x9cA new robust method for six-port reflectometer calibrationxe2x80x9d which has been submitted to the IEEE transaction.
The disadvantage of all those known hardware calibrations is the inherently required cutting-off of the physical connections in order to assemble particular termination, which is in the case of receiver applications absolutely impractical.
Recently a calibration procedure for a six-port coherent direct receiver without physical disconnection of the system has been disclosed in J. Li, R. G. Bosisio and K. Wu xe2x80x9cDual-tone calibration of six-port direct digital millimetric receiverxe2x80x9d, IEEE Trans. Microwave Theory Technique, vol. 44, pages 93-99, January 1996. However, this known technique requires complicated monitoring of the outputs, large observation time and the alternation (change) of the local oscillator (see reference 18 in FIG. 4) level. In this document it is disclosed two use simply two different frequencies without any modulation and to supply them to the input ports of a six-port receiver.
There is a plurality of requirements for a calibration procedure for a direct receiver based on a six-port structure:
The calibration should be effected without physical disconnection of the system using the same programmed sampling rate as for the data transmission,
the time for calibration should be as short as possible and
the required computational effort for calibration coefficients should be minimized and adapted to fast hardware digital computation units.
Therefore it is an object of the present invention to provide a calibration procedure generally for n-port receivers, which calibration procedure satisfies the above-mentioned conditions.
The basic idea of the present invention is to use a calibration sequence for direct n-port receiver structures with coherent or non-coherent demodulation. Said pre-defined calibration sequence is fed to the input of the direct receiver. A carrier frequency is used and a pre-defined sequence is applied as the modulation sequence of the carrier frequency.
According to the present invention therefore a method for calibrating a n-port receiver is proposed, said n-port receiver comprising a passive circuit with two inputs, at least one input being supplied with a high-frequency signal to be measured, and at least three outputs supplying power levels for a signal processing unit, which signal processing unit calculates a complex signal based on the at least two power levels and calibration coefficients. A predetermined calibration sequence with different symbols is fed to the at least one input for the signal to be measured and the calibration coefficients are calculated based on the calibration sequence.
When referencing to n-port receivers in the following description, n is an integer which can assume the value of four, five and six. As the case of n equal to six is known in the prior art, the following description is made with reference to a six-port receiver. However, the present invention is by no means limited on the case of n equal to six. The calibration of a n-port receiver is essentially independent of the fact whether n is four, five or six.
The calibration sequence can be a modulated RF signal.
The calibration sequence can comprise at least five different states (symbols).
The calibration sequence can be a n-PSK modulated signal.
The number of different symbols in the calibration sequence can be N and the minimum phase distance between two symbols of the calibration sequence can be 2xcfx80/N.
The time duration of the calibration sequence can be at least five time sample periods of the six-port receiver.
Usually a predetermined (known) modulation scheme will be used. In this case the calibration sequence can comprise preferably only symbols corresponding to modulation states of the predetermined modulation scheme.
Particularly the calibration sequence can comprise symbols corresponding to all modulation states of the predetermined modulation scheme. A calibration vector can be calculated on the basis of a matrix representing the modulation states of the predetermined modulation scheme and a matrix of the values detect by the six-port receiver.
The correction values of the output signals of the six-port receiver can then be calculated on the basis of the calibration vector.
The calibration sequence can be fed repeatedly to an input of the six-port receiver.
The calibration sequence can be generated by a remote transmitter.
Alternatively, the calibration sequence can be generated by an additional hardware block connected to the six-port receiver. The additional hardware block can be for example a RF source modulated by the predefined calibration sequence.
As a further alternative the calibration sequence can be generated by a local transmitter, which is particularly advantageous in the case of a TDD transmission system because of the identical operation frequency.
The calibration sequence generated by the local transmitter can be supplied to the six-port receiver without power amplification.
The present invention furthermore proposes the use of a calibration method as mentioned above, wherein the six-port receiver is comprised in a mobile telecommunications device.
The present invention furthermore relates to a receiving apparatus. The receiving apparatus comprises a n-port receiver with a passive circuit with two inputs, at least one input being supplied with a high-frequency signal to be measured, and at least three outputs supplying power levels for a signal processing unit which calculates a complex signal based on the at least two power levels and calibration coefficients. The receiving apparatus furthermore comprises means for supplying a predetermined calibration sequence with different symbols to the at least one input for the signal to be measured. The signal processing unit calculates the calibration coefficients based on the calibration sequence.
The n-port receiver can be a six-port receiver.
The calibration sequence can be a modulated RF signal.
The calibration sequence can comprise at least five different states (symbols).
The calibration sequence can be a PSK modulated signal or a D (differential) PSK modulated signal.
The number of different symbols in the calibration sequence can be N and the minimum phase distance between two symbols of the calibration sequence can be 2xcfx80/N.
The duration of the calibration sequence can be at least five time sample periods of the six-port receiver.
The means for supplying the calibration sequence can be a remote transmitter.
Alternatively, the means for supplying the calibration sequence can be an additional hardware block connected to the six-port receiver.
As a further alternative the means for supplying the calibration sequence can be a local transmitter.
According to the present invention furthermore a receiving apparatus according to any of the proceeding claims is proposed.
Preferred embodiments of the invention will now be explained referring to the figures of the annexed drawings.